Polymers find uses in a variety of plastic articles including films, sheets, fibers, foams, molded articles, adhesives and many other specialty products. For applications in the areas of packaging, agriculture, household goods and personal care products, polymers usually have a short (less than 12 months) use cycle. For example, in food packaging, polymers play the role of a protective agent and are quickly disposed of after the contents are consumed. Household products such as detergent bottles and diapers are immediately discarded once the product is used.
The majority of this plastic material ends up in the solid waste stream, headed for rapidly vanishing and increasingly expensive landfill space. While some efforts at recycling have been made, the nature of polymers and the way they are produced and converted to products limits the number of possible recycling applications. Repeated processing of even pure polymers results in degradation of material and consequently poor mechanical properties. Different grades of chemically similar plastics (e.g., polyethylenes of different molecular weights, as used in milk jugs and grocery sacks) mixed upon collection can cause processing problems that make the reclaimed material inferior or unusable.
Absorbent article applications such as diapers, sanitary napkins, pantiliners and the like, involve several different types of plastics. In these cases, recycling is particularly costly because of the difficulty in separating the different components. Disposable products of this type generally comprise some sort of fluid-permeable topsheet material, an absorbent core, and a fluid-impermeable backsheet material. Heretofore, such absorbent structures have been prepared using, for example, topsheet materials prepared from woven, non-woven, or porous formed-film polyethylene or polypropylene materials. Backsheet materials typically comprise flexible polyethylene sheets. Absorbent core materials typically comprise wood pulp fibers or wood pulp fibers in combination with absorbent gelling materials. Although such products largely comprise materials which would be expected ultimately to degrade, and although products of this type contribute only a very small percentage of the total solid waste materials generated by consumers each year, nevertheless, there is currently a perceived need to devise such disposable products from materials which are compostable.
A conventional disposable absorbent product is already to a large extent compostable. A typical disposable diaper, for example, consists of about 80% of compostable materials, e.g., wood pulp fibers, and the like. In the composting process soiled disposable absorbent articles are shredded and commingled with organic waste prior to the composting per se. After composting is complete, the non-compostable particles are screened out. In this manner even today's absorbent articles can successfully be processed in commercial composting plants.
Nevertheless, there is a need for reducing the amount of non-compostable materials in disposable absorbent articles. There is a particular need to replace polyethylene backsheets in absorbent articles with liquid impervious films of compostable material, because the backsheet is typically one of the largest non-compostable components of a conventional disposable absorbent article.
In addition to being compostable, the films employed as backsheets for absorbent articles must satisfy many other performance requirements. For example, the resins should be thermoplastic such that conventional film processing methods can be employed. These methods include cast film and blown film extrusion of single layer structures and cast or blown film coextrusion of multilayer structures. Other methods include extrusion coating of one material on one or both sides of a compostable substrate such as another film, a non-woven fabric, or a paper web.
Still other properties are essential in product converting operations where the films are used to fabricate absorbent articles. Properties such as tensile strength, tensile modulus, tear strength, and thermal softening point determine to a large extent how well a film will run on converting lines.
In addition to the aforementioned properties, still other properties are needed to meet the end user requirements of the absorbent article. Film properties such as impact strength, puncture strength, and moisture transmission are important since they influence the absorbent article's durability and containment while being worn.
Once the absorbent article is disposed of and enters a composting process, other properties become important. Regardless of whether incoming waste is preshredded or not, it is important that the film or large film fragments undergo an initial breakup to much smaller particles during the initial stages of composting. Otherwise, the films or large fragments may be screened out of the compost stream and may never become part of the final compost.
In the past, the biodegradability and physical properties of a variety of polyhydroxyalkanoates (PHAs) have been studied. Polyhydroxyalkanoates are polyester compounds produced by a variety of microorganisms, such as bacteria and algae. While polyhydroxyalkanoates have been of general interest because of their biodegradable nature, their actual use as a plastic material has been hampered by their thermal instability. For example, poly-3-hydroxybutyrate (PHB) is a natural energy-storage product of bacteria and algae, and is present in discrete granules within the cell cytoplasm. However, unlike other biologically synthesized polymers such as proteins and polysaccharides, PHB is thermoplastic having a high degree of crystallinity and a well-defined melt temperature of about 180.degree. C. Unfortunately, PHB becomes unstable and degrades at elevated temperatures near its melt temperature. Due to this thermal instability, commercial applications of PHB have been extremely limited.
As a result, investigators have studied other polyhydroxyalkanoates such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), in the hopes of discovering a polyhydroxyalkanoate having sufficient thermal stability and other suitable chemical and physical properties for use in practical applications. Unfortunately, polyhydroxyalkanoates such as PHB and PHBV are difficult to process into films suitable for backsheet applications. As previously discussed, the thermal instability of PHB makes such processing nearly impossible. Furthermore, the slow crystallization rates and flow properties of PHB and PHBV make film processing difficult. Examples of PHB homopolymer and PHBV copolymers are described in U.S. Pat. No. 4,393,167, Holmes et al., issued Jul. 12, 1983, and U.S. Pat. No.4,880,592, issued Nov. 14, 1989. PHBV copolymers are commercially available from Imperial Chemical Industries under the tradename BIOPOL. PHBV copolymers are currently produced with valerate contents ranging from about 5 to about 24 mol %. Increasing valerate content decreases the melt temperature, crystallinity, and stiffness of the polymer. An overview of BIOPOL technology is provided in BUSINESS 2000+ (Winter, 1990).
Due to the slow crystallization rate, a film made from PHBV will stick to itself even after cooling; a substantial fraction of the PHBV remains amorphous and tacky for long periods of time. In cast film operations, where the film is immediately cooled on chill rolls after leaving the film die, molten PHBV often sticks to the rolls restricting the speed at which the film can be processed, or even preventing the film from being collected. In blown films, residual tack of the PHBV causes the tubular film to stick to itself after it has been cooled and collapsed for winding.
U.S. Pat. No. 4,880,592, Martini et al., issued Nov. 14, 1989, discloses a means of achieving a PHBV monolayer film for diaper backsheet applications by coextruding the PHBV between two layers of sacrificial polymer, for example a polyolefin, stretching and orienting the multilayer film, and then stripping away the polyolefin layers after the PHBV has had time to crystallize. The remaining PHBV film is then laminated to either water soluble films or water insoluble films such as polyvinylidene chloride or other polyolefins. Unfortunately, such drastic and cumbersome processing measures are necessary in an attempt to avoid the inherent difficulties associated with processing PHBV into films.
Based on the foregoing, there is a need for plastic articles that can biodegrade. In effect such biodegradable articles would facilitate the "recycling" of plastic articles into another usable product, topsoil, through composting. To satisfy this need, there is a preliminary need for a biodegradable polymer which is capable of being easily processed into a plastic article for use in a disposable product.